1. Field of the Invention
The present invention relates to metal-coated cubic boron nitride abrasive grains. More particularly, the present invention relates to metal-coated cubic boron nitride abrasive grains in which bonding strength between the cubic boron nitride abrasive grains and a metallic coating layer is increased, relates to a method for producing the metal-coated cubic boron nitride abrasive grains, and relates to a resin bonded grinding wheel incorporating the metal-coated cubic boron nitride abrasive grains.
2. Description of Related Art
Cubic boron nitride (which hereinafter may be simply referred to as “cBN”) abrasive grains have been employed as abrasive grains of, for example, grinding wheels and coated abrasives, since cBN exhibits hardness second only to diamond, and better chemical stability against an iron workpiece when compared with diamond. The cubic boron nitride abrasive grains are produced through treatment of hexagonal boron nitride (which may be referred to as hBN) in the presence of a catalytic substance under high-temperature, high-pressure conditions (for example, about 1,400° C. to about 1,600° C., and about 4 GPa to 6 GPa), under which cubic boron nitride is thermally stable.
The thus-produced cubic boron nitride abrasive grains are employed in, for example, electroplated grinding wheels, metal bonded grinding wheels, vitrified bonded grinding wheels, and resin bonded grinding wheels.
Among these grinding wheels, the resin bonded grinding wheels, in which resin is used as a bonding agent, have a problem in that more of the cubic boron nitride abrasive grains come off during grinding operations when compared with other grinding wheels, and thereby the grinding ratio is reduced (i.e., the operative life of the grinding wheel is reduced). It is believed that the coming-off of the abrasive grains is mainly due to the following two reasons: (a) retention force between a bonding layer and a surface of a coating, such as a metallic coating, which is coated on the surface of the abrasive grains, is small; and (b) retention force between a coating, such as a metallic coating, and the surface of cubic boron nitride abrasive grains is small; therefore, various efforts have been made to find measures to increase retention force of abrasive grains so that the operative life of the grinding wheel is prolonged.
For example, metal-coated abrasive grains have been developed and are currently employed in resin bonded grinding wheels, in which a single-layered or multi-layered metallic coating, such as of nickel, nickel-phosphate, cobalt, or cobalt-phosphate, is applied to the surface of the abrasive grains so that retention force in the bonding layer is increased due to irregularities of the surface of the metallic coating (Patent Documents 1 to 4).
Patent Document 1 discloses a method for producing nickel-coated abrasive grains having high retention force due to surface irregularities, in which, during the formation of a metallic layer on the surface of the abrasive grains, spongiform nickel is used as a first layer, and dense nickel is used as a second layer.
Patent Document 2 discloses a resin bonded grinding wheel exhibiting a grinding ratio better than that of conventional ones in which a coating, which includes nickel as a first layer, cobalt as a second layer, and nickel as a third layer, is applied.
By applying a coating to the surface of the abrasive grains as explained above, retention force of the abrasive grains in the bond can be increased, the amount of the abrasive grains coming off during grinding operation can be reduced, and thus the grinding ratio of the grinding wheel has been improved.
However, even though the retention force between the metal-coated abrasive grains and the resin bond is increased, the retention force between the metallic coating and the abrasive grains become comparatively small, and the abrasive grains tend to come off while the metallic coating is left in the bonding layer, and thereby a problem is still encountered in that sufficient operative life of the grinding wheel cannot be ensured.
In view of the above circumstances, metal-coated abrasive grains exhibiting an increased retention force between a metallic coating and abrasive grains have been developed, in which a layer among two or more layers of a metallic coating is chemically bonded to the surface of the cubic boron nitride abrasive grains (Patent Documents 5 to 8).
For example, Patent Document 5 discloses metal-coated abrasive grains in which a metallic coating, such as of tungsten, molybdenum, tantalum, or niobium, is applied to the surface of diamond abrasive grains or cubic boron nitride abrasive grains so as to form a solid solution due to chemical bonding or diffusion.
Moreover, Patent Document 6 discloses metal-coated abrasive grains in which a double-layer structure is employed, and an interstitial metallic layer, serving as a first layer, is formed and chemically bonded to the surface of cubic boron nitride grains using, for example, a salt bath deposition process, a chemical vapor deposition process, or a physical vapor deposition process, and subsequently a metallic layer, serving as a second layer, is formed using, for example, an electroless deposition process, an electrolytic deposition process, or a vapor deposition process.
According to the above Patent Documents, the coming-off of the abrasive grains can be prevented by increasing the retention force due to chemical bonding between the surface of the abrasive grains and the metallic coating or due to forming of the interstitial metallic layer.
Patent Document 7 discloses abrasive grains (compound abrasive grains) in which the abrasive grains are bonded to each other using a vitrified bonding agent or a metallic bonding agent. According to the Document, because a vitrified bond or metallic bond is used for coating, the retention force between the coating and the surface of the abrasive grains is increased, the abrasive grains are strongly bonded to each other, and the retention force in the resin bond is also increased due to irregularities on the compound abrasive grains.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. S60-51678    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. S59-142066    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. S59-30671    Patent Document 4: Japanese Unexamined Patent Application, First Publication No. S60-52594    Patent Document 5: Japanese Unexamined Patent Application, First Publication No. H04-18567    Patent Document 6: Japanese Unexamined Patent Application, First Publication No. H05-194939    Patent Document 7: Japanese Unexamined Patent Application, First Publication No. H10-337670    Patent Document 8: Japanese Unexamined Patent Application, First Publication No. H09-323046
As described above, it is believed that the coming-off of the abrasive grains is mainly due to the following two reasons: (a) retention force between a bonding layer and a surface of coating, such as a metallic coating, which is coated on the surface of the abrasive grains, is small; and (b) retention force between a a coating, such as a metallic coating, and the surface of cubic boron nitride abrasive grains is small. Even though the retention force of the metal-coated abrasive grain in the bonding layer is increased according to the methods disclosed in, for example, Patent Documents 1 to 4, the amount of the abrasive grains coming off cannot be efficiently reduced because the retention force between the coating and the abrasive grains is small. Accordingly, Patent Documents 5 to 8 have proposed the methods for increasing the retention force between the coating and the surface of the abrasive grains. According to these methods, the retention force between the coating and the surface of the abrasive grains is increased, the retention force of the grinding agents in the resin bond is increased, and thus the grinding ratio is improved. However, according to these methods, bonding between the coating and the surface of the abrasive grains is so strong that the coming-off of the abrasive grains in which cutting edges have been worn out is also restrained; therefore, a problem is encountered in that grinding power during grinding operations is increased.
In the case of the resin bonded grinding wheels, when the grinding power becomes high, deterioration of the resin bond and burning of a workpiece tend to easily occur due to grinding heat. It is necessary for the abrasive grains to appropriately come off for obtaining sharp cutting edges in order to maintain superior grinding performance without increasing the grinding power.
While it is strongly demanded that the retention force of the grinding agents be increased (i.e., the grinding ratio be increased) in view of the operative life of tools and overall machining cost, it is also necessary for the abrasive grains to appropriately come off for obtaining sharp cutting edges in order to maintain superior grinding performance without increasing the grinding power.
The methods disclosed in Patent Documents 5 to 8 contain problems in that the production cost is increased due to complicated means and steps for forming the coating, and it is difficult to control the mass ratio of the coating so to be constant.
In view of the above-described problems, new abrasive grains for resin bonded grinding wheels have been strongly demanded by which an increase in the grinding ratio and restraint of increasing grinding power can be simultaneously achieved.